Why do so many contemporary humans suffer from a mental illness? The global, collective prevalence of chronic depression, Attention-Deficit Disorder (ADD), Obsessive-Compulsive Disorder (OCD), autism, and other disorders of the mind is today fairly high, which is very concerning, seeing as these types of conditions in many cases have a devastating impact on measures of health and quality of life.
Given that so many people in the world today don’t operate at peak capacity, mental health wise, it’s not surprising that a substantial amount of time, money, and resources are put into developing effective treatments for various psychiatric illnesses. In this very moment, researchers are hard at work trying to figure out which receptors and genes that play a critical role in the development of Alzheimer’s disease, what types of pharmaceutical compounds that are useful in the treatment of autism, and how gut bacteria may contribute to the development of Parkinson’s disease, among other things.
This type of research can certainly be useful, in that it may help aid our progress towards developing therapeutic solutions to various mental health disorders. It has its limitations though. One of the things it won’t tell us is why mental disorders such as the ones mentioned in the first paragraph exist in the first place. In order to answer that question, we need to bring out our Darwinian toolkits, which can help us determine what the ultimate causes of mental illness are; an understanding that can potentially be turned into effective treatments for OCD, ADD, autism, and other similar conditions.
Evolutionary mismatch: The ultimate cause of mental illness?
The fact that conditions such as major depression and autism exist is interesting, from a Darwinian point of view, considering that they severely impair Darwinian fitness. This is particularly true if the affected organism lives in a natural environment. A severely depressed hunter-gatherer would obviously be at a disadvantage in the struggle for existence when compared with a healthy, thriving hunter-gatherer. Not only would the former likely be less motivated to optimize his food seeking behavior, but his affliction would probably compromise his ability to develop tight social bonds and attract a healthy mate. In other words, he would be less likely to survive and reproduce.
The fact that disorders such as major depression compromise organismal fitness begs the question: Why hasn’t natural selection eliminated – or at least severely reduced the frequency of – well-known risk alleles for major depression and other mental health conditions that compromise organismal fitness?
As I see it, there is only one good answer to this question, and that is that the relevant alleles only become problematic under conditions that differ from the conditions under which we evolved. Otherwise, they wouldn’t have been as common as they are. This idea is supported by a large body of research, including reports from explorers and researchers which indicate that hunter-gatherers and other non-westernized, traditional people tend to be healthy, vibrant, and content with life (1, 2, 3, 4); studies showing that the prevalence of many mental health conditions has increased as of late in countries that have become increasingly more westernized (2, 5); and a large body of evidence implicating diet and lifestyle practices in the pathogenesis of various mental illnesses (2, 5, 6, 7, 8, 9).
The adaptive value of ancient brain features
Darwinian logic suggests that the bodily features that leave us susceptible to develop mental illness in a modern environment initially evolved because they serve an adaptive function. Let’s take chronic depression for example. Recent research has made it clear that inflammation is a fundamental cause of many cases of depression (6, 10, 11). Not only do depressed people tend to have elevated levels of proinflammatory cytokines, acute phase proteins, chemokines, a